INTRODUCTION

The most serious problem being experienced globally in the present age is the pollution of the environment mainly by inorganic, organic, and organometallic materials¹. According to the World Health Organization (WHO), 42% of sub-Sahara Africa lacks safe drinking water ². It is also predicted that more than 47% of the global population will experience severe water shortage by 2030. Rivers are essential and vulnerable freshwater resources that are critical for sustaining all life forms ³. The economic, social and political advancement of nations is mostly related to the availability of fresh water ⁴. Rivers play an important role in assimilating and carrying municipal and industrial waste water, runoff and manure discharge which are the primary causes of river pollution⁵. Trace metal contaminations are important because of their potential toxicity to the environment and human beings⁶. Some metals like Cu, Mn, Fe and Zn are essential micronutrients required for life processes in plants and animals while many other metals such as Pb, Cd and Cr have no known physiological functions^7,8. Metal-induced toxicity symptoms in human beings range from milder ones such as nausea, excessive salivation, vomiting, abdominal pain, diarrhea with bleeding, dizziness, dermatitis, headaches, aggressiveness and hallucinations to very serious ones such as endocrine disruption, lung disease, liver and kidneys failure, hepatic damage, mutagenic, carcinogenic and teratogenic effects⁹. Assessment of water quality parameters provides the basis for determining the suitability of water for its designated uses and to improve existing conditions ¹⁰. Nairobi River flows through the residential and industrial areas of Nairobi City. It receives and drains untreated and treated discharges of various types. Water from this river is used for agriculture and domestic purposes especially by people living downstream. Nairobi City was established in the early 1900s with a population of about 250,000 ¹¹. It was a stop–over point for the then Kenya-Uganda Railway workers ¹². At that time, Nairobi boasted of good environmental health, and it was reputed as the green city in the sun ¹¹. This reputation has changed as a result of rapid urbanization and population growth. In 2009, the population of Nairobi City was about 3.1 million¹³. Following the inadequate handling of waste management, residents do not dispose their solid waste appropriately. As a result, people dump randomly within the local environment including near the river banks. The consequence is that such rivers may be highly polluted with human waste alongside other domestic waste. Accordingly, pollution extends its influence to the international environment, since the waste flows downstream and pollutes larger rivers at junctions, which may ultimately pour its contents into the Indian Ocean, effectively dispensing the waste across the East African coastline and thus damaging the marine environment ¹².

Most adverse pollution sources of natural waters have been attributed to industrial and agricultural effluents ¹⁴. However, measures and standards have been put in place to ensure that industries treat their effluent before releasing it to the sewer. This may not necessarily be the case, since effluents from domestic and other unclassified sources may still pollute pristine waters usually recommended for drinking and domestic use¹⁵. Therefore this study focuses on the water quality of Nairobi River which flows through some residential areas in the city for various physico-chemical parameters and heavy metals considered well-established eco-toxicants and xenobiotic.

MATERIALS AND METHODS:

The reagents used in this study were of purity ≥ 99%. All reagents silver nitrate, potassium chromate, sodium chloride, Sulfaver 4 sulphate reagent, nitric acid, Nitriver 3 nitrite reagent, molybdate and amino acid reagents, fluoride, nitrite, phosphate and sulphate standard solutions, lead, copper, manganese, chromium, iron and zinc standard solutions were purchased from Sigma Aldrich, Inc., St. Louis, Missouri, USA.

Description of the study area

Nairobi River, which is the focus of this study, has its source at Ondiri swamp near Kikuyu Township and empties its water into the Indian Ocean (cf. Fig 1). Six sampling points selected for this study lie between longitudes 036.7358^oto 036.8094^oE, latitude 01.2744^o to 01.29965^oS and altitude 1626-1816 meters above the sea level (m.as.l). Chiromo Kirichwa Kubwa (CKK) is located in the University of Nairobi at Chiromo Campus, Kirichwa Ndogo (KN) runs through a residential area and there were potted plants near this point during the time of sampling. Kileleshwa Kirichwa Kubwa (KKK) is located near a residential area and there is also a filling station, car wash and a garage near the sampling point. Lenana School (LS) and Lenana dam (LD) are located in Lenana area where there was cultivation taking place in the school farm and a piggery. Riara (RR) is located near a residential area.

Sample collection

Water samples were collected from six sampling points during the rainy season (November) and the dry season (June). At each the sampling point, sampling containers (cleaned and rinsed with deionized water) were rinsed twice with the water sample before a sample was collected. The samples were collected in duplicate and concentrated nitric acid was added to water samples for metal analysis in order to preserve the water samples. The water temperature, pH, dissolved oxygen (DO) and total dissolved solids (TDS) were measured in situ using a portable EC/TDS/NaCl/Temp/pH meter (model MI 306) and DO meter (model HI9146) respectively as soon as the samples were collected . The samples were transported to the laboratory and refrigerated at a temperature of 4^oC while waiting for analysis.

Laboratory analysis

Standard methods used for chemical analysis are incubation method as BOD₅ (BOD), reactor digestion method (COD), gravimetric method (Total Suspended Solids), amino acid method (phosphate), sulfaver 4 method (sulphate), diazotization method (nitrite) and titration method (fluoride and chloride). The determination of heavy metals (Cu, Fe, Pb, Mn, Cr and Zn) was carried out using flame Atomic Absorption Spectrophotometer (AAS) model SHIMADZU 6300 after acid digestion with concentrated nitric acid ¹⁶. The calibration for each heavy metal was carried out by serial dilution of 1000 mg/L commercial stock solutions. The calibration standard solutions were between 0.5 and 5 mg/L. A spectrophotometer model DR 3900 was used to determine the levels of chemical oxygen demand, sulphate, phosphate and nitrite ¹⁷.

RESULTS AND DISCUSSION:

The pH values obtained in this study ranged from 6.89 to 7.77 which are within the acceptable World Health Organization (WHO) and Kenya Bureau of Standards (KEBS) limit for natural water (6.0-8.5) ¹⁸. High pH values recorded at Kirichwa Ndogo (KN) (7.64 ± 0.19) may be attributed to fertilizers from the potted plants near the sampling point. Effluents containing acid from car wash and garage may also result in lower pH values at Kileleshwa Kirichwa Kubwa (KKK) (6.89) during the dry season. The temperature values were observed to be slightly lower during the dry season (18.3-20.4^oC ±0.05^oC) as compared to the wet season 19.2- 25.0 ⁰C ± 0.05 ^oC. The water temperature depends on the time of sampling, season and the temperature of effluent which are discharged into the river. The electrical conductivity values were above the WHO limits of 600µS/cm for natural water at Lenana School (LS) and Riara (RR) during both the dry and the wet seasons, and Kileleshwa Kirichwa Kubwa (KKK) during the wet season. This was attributed to domestic effluent discharges into the river which increased the concentration of the ions. Similar observation was also made for Bertam River in Malaysia¹⁹. The variation in electrical conductivity is presented in Fig. 2. Clearly, the section around Lenana school had the highest electrical conductivity and this may be attributed to possible effluent discharge from the school laboratory which has high levels of ionic species.

Fig 2: The mean concentration of electrical conductivity as measured along Nairobi River

The trend for Total Dissolved Solids (TDS) values was similar to that observed for electrical conductivity. This is expected, since most dissolved solids in water are ionic species which tend to increase electrical conductivity. Therefore, TDS values predictably increase with increase in electrical conductivity. TDS values were below the acceptable National Environmental Management Authority (NEMA, Kenya) limits of 1200 mg/L for natural water and the range was 176-438 mg/L (cf. Fig 2). Runoff or leachate which carries dissolved minerals from the school farm into the river could have contributed to high TDS values at Lenana School (438 mg/L) during the wet season. Similar observation was also made in Bertam River in Malaysia¹⁹. The proximity of Lenana and Riara Schools to the river could have contributed to relatively high TDS values at these sampling points. TDS is not a health hazard although high levels may indicate hard water which may lead to scale build up in pipes and aesthetic problems such as salty or bitter taste in water. The concentration of total suspended solids (TSS) was observed to be significantly higher during the wet season as compared to the dry season. This variation was attributed to runoff which carries particles into the river during the wet season. Similar observation was made by ¹⁹ for Bertam River in Malaysia. The high TSS values recorded at Lenana dam (LD) during the wet season was attributed to ploughing which had been carried out in the school farm. Lenana School contained the least concentration of TSS which was as a result of vegetation near the river which effectively reduced soil erosion into the river. All the sampling points were exposed to particle pollutant since TSS concentrations were above the NEMA recommendation of 30 mg/L for domestic water (cf. Fig 3). High TSS values may lead to gastrointestinal irritations ²⁰. The level of chemical oxygen demand (COD) was found to be higher during the dry season (90-730 mg O₂/L) as compared to the wet season (40-410 mg O₂/L). This variation was attributed to a higher concentration of chemical species because of reduced volume of water during the dry season. The high concentration of COD recorded at Lenana School (730 mg/L) during the dry season was attributed to animal waste from the pig farm in the school; since the most substances oxidized by dissolved oxygen is organic matter having biological origin like dead plants and animal wastes ²¹. The COD levels in all the sampling points were above the NEMA limits of 50 mg/L for discharge into natural water courses.

Fig 3: The mean concentration of COD, TDS and TSS recorded along Nairobi River

The dissolved oxygen (DO) concentrations were found to be higher during the wet season (17.23-24.29 mg/L), as compared to the dry season. This is probably due to increased volume of water during the wet season hence high aeration due to turbulence brought about by storm water. During the dry season, the water volume was less leading to minimal aeration and less dissolved oxygen in the water. This DO profile may also be attributed to relatively higher BOD levels during the dry season as compared to the wet season (cf. Fig 4); the higher the BOD level, the more rapidly oxygen is depleted thereby resulting to low DO levels. The dissolved oxygen concentration range was 8.10-22.72 mg/L, which is within the acceptable limit for domestic water (≥4 mg/L) by the World Health Organization. The Biochemical Oxygen Demand (BOD) was found to be slightly higher during the dry season as compared to the wet season (cf. Fig 3). This variation was attributed to dilution of water during the wet season and sedimentation process during the dry season. This observation agrees with the works of ^{22, 23, 24} for Aiba stream and Tarkwa bay in Nigeria and Dandora treatment plant in Nairobi respectively. High levels of BOD recorded at Lenana dam and Kirichwa Ndogo was attributed to chemical usage of fertilizers from the school farm and the potted plants respectively. At Riara sampling point, the high BOD level was attributed to domestic effluent discharge into the river and also organic waste. The values of BOD for both seasons were found to be below the NEMA limit of 30 mg/L for effluent discharge into natural water courses and above the WHO limit of 10 mg/L for domestic water.

Fig 4: The levels of DO and BOD recorded along Nairobi River

Anions

The concentration of phosphates increased during the wet season as compared to during the dry season. Similar observation was made by ²² for Aiba stream in Nigeria. The high levels of phosphates during the wet season could be attributed to the high rate of decomposition of organic matter and from surface water runoff. The concentration of phosphate Lenana School was above the WHO recommended limit of 5 mg/L for natural water. This was attributed to surface runoff as farming activities occurred during both seasons or due to use of detergents. While the source of phosphates is usually attributed to the use of fertilizers, man-made sources such as domestic discharges especially use of phosphate-based detergents in households for laundry ²⁵ or even changes in land use in areas where phosphorus is naturally abundant in the soil contributes also to phosphates in the water.

Fig 5: The concentration of anion species as determined in Nairobi River

The concentration of sulphates was found to be much higher during the wet season as compared to the dry season. This variation might be attributed to surface runoff from soils containing minerals that are rich in sulphate. Similar observation was made by ²⁵ for three selected springs in Nandi County, Kenya. Sulphate was not detected in Lenana dam during the dry season. This might be due to self-purification process taking place in the dam and also, there could be no point source. Riara recorded the highest concentration of sulphate (68 mg/L) during the wet season. This might be attributed to a point source for example household wastes like detergents. The levels of sulphate were higher than all the other anions (cf. Fig 5) indicating that there might be a point source. There is no health-based guideline value recommended for sulphate, however intake of high sulphate levels has gastrointestinal effects. The fluoride concentration was above the NEMA guideline value of 1.5 mg/L for domestic water and effluent discharge into the environment in Riara, Lenana School and Lenana dam during both dry and wet seasons. High concentrations of fluoride at Lenana dam and Lenana School might be due to contamination by fertilizers and animal wastes. Similar observations were made by ²⁰. Concentration of fluoride above 1.5 mg/L carry an increasing risk of dental fluorosis, and much higher concentrations lead to skeletal fluorosis. The concentration of chloride was observed to be higher during the wet season (0.085-0.886 mg/L) rather than the dry season (0.01-0.071 mg/L). This variation might be attributed to surface runoff from soils contaminated by chlorides during the wet season. Anthropogenic sources of chlorides include livestock waste, human sewage and synthetic fertilizers ²⁷. There is no health- based guideline value that is recommended for chloride in drinking water; however, chloride concentrations in excess about 250 mg/L can give rise to a detectable taste in water ²⁷. The concentration of nitrites was below the NEMA guideline of 3 mg/L for domestic water in all the sampling points. Its concentration was observed to be higher during the wet season (0.004-0.585 mg/L) as compared to the dry season (0.002-0.373 mg/L).

Heavy metals

The concentration of manganese was higher during the dry season in comparison to the wet season (Table 1). This variation may possibly be attributed to the dilution of water during the wet season. High concentrations of Mn at Riara might be associated with waste water discharges containing bleaching agents and disinfectants from residential houses constructed close to the river. The concentration of manganese in all the sampling points with the exception of Kirichwa Ndogo (KN) during the wet season was above the KEBS limits of 0.1 mg/L for domestic water. This is a point of concern since high concentrations of manganese have been shown to have health effects such as liver damage, neurotoxicity, chronic respiratory inflammation and birth defects such as cleft lip, heart defects, imperforate anus and deafness, in addition to aggressive behaviour and disturbances in libido ⁹. These symptoms tend to persist even after Mn body burden returns to normal. The mean concentration of iron in the water samples remained above the KEBS critical limit of 0.3 mg/L for domestic water ¹⁸ in all the sampling points except Kirichwa Ndogo (KN). Iron is profuse in the earth’s crust and has many uses industrially and that is probably the reason why it was present in relatively high quantities in all the sampling points. The concentration of copper was higher during the rainy season as compared to the dry season. This might be attributed to the persistence of copper complexes in the sediments which dissolved when the volume of water increased during the rainy season and therefore were detected in the water. There was no metal pollution associated with copper because its concentration was below the WHO and KEBS critical limits in all the sampling points. The concentration of zinc in all the sampling points was found to be below the NEMA limit of 1.5 mg/L for domestic water and 0.5 mg/L for effluent discharge into the environment. These values are far much lower than the 30.58-85.64 mg/L obtained by ²⁸ on another section of Nairobi River. Its concentration was higher during the wet season as compared to the dry season. This may be attributed to surface runoff from soils that may contain natural traces of zinc or dissolution of zinc from the sediments. Leachate from fertilizers and insecticides, urban runoff and municipal sewage are the main sources of zinc in waters ²⁹. The concentration of lead was above the NEMA limit of 0.01 mg/L for effluent discharge into the environment in all the sampling points except Riara and Kirichwa Ndogo. The high concentration of lead recorded at Kileleshwa Kirichwa Kubwa (KKK) might be attributed to effluents from the garage and car wash situated close to the river. The higher concentration of lead obtained during the wet season was attributed to the surface runoff from a point source. For example, lead present at Lenana School and Lenana dam (LD) during the wet season might be attributed to runoff of lead from paint since the school buildings had been renovated during the period for the second sampling. The concentrations of lead in this study were lower than the 0.2-7.5 mg/L (Table 1) reported by ³⁰ on another section of Nairobi River. Because of its ubiquity and toxicity even at low concentrations, lead is one of the most insidious of all environmental hazards ⁹. Chromium was not detected during the wet season, a factor which was attributed to dilution of water. During the dry season, the concentration of chromium in Chiromo Kirichwa Kubwa and Lenana dam was above the KEBS provisional guideline value of 0.05 mg/L for natural water. The source of chromium at Lenana School and Lenana dam was attributed to effluents containing detergents.

It is evident that most of the pollution originated from storm water runoff from residential units therefore it is recommended that biofiltration swales and rain gardens (vegetated filtering systems used in treating storm water runoff) should be constructed along the landscapes of residential sites. The concentrations of heavy metals in the river may be reduced by planting Azolla on waterways leading to the river as recommended elsewhere ³¹. Azolla has the capability of accumulating large quantities of heavy metals such as Cu, Fe, Mn, Zn, Pb and Sr ³¹. Water hyacinth (Eichhornia Crassipes) has also been found to be effective in reducing the levels of heavy metals like Cd, Cr, Ni, Co, Pb and Hg ^{32, 33}, however it is considered as a pest because it spreads fast and therefore creates good breeding conditions for mosquito vectors of malaria, encephalitis and filariasis as a result of low oxygen conditions beneath the mats of vegetation ³⁴. For the removal of nutrients, Melaleuca ericifolia, Carex appressa, Juncus flavidis and Juncus amabilis plants have proven effective ³⁵.

CONCLUSION:

This investigation has shown that pH, TDS, DO and temperature values of Nairobi River are acceptable for a pristine river. Moreover, the levels of anions were below the WHO and National Environmental Management Authority (NEMA, Kenya) permissible limits in all the sampling points except for fluoride and phosphates in some sampling points. The concentrations of Zn and Cu were below WHO standards. Therefore, there was no metal pollution that can be attributed to these elements in the selected sampling points. Isolated cases of pollution by Cr were recorded during the dry season. The concentrations Mn and Fe were above the critical limits set by the Kenya Bureau of Standards (KEBS) in all the sampling points except for Kileleshwa Kirichwa Kubwa during the wet season and Kirichwa ndogo during the dry season. The concentration of Pb was above the WHO and KEBS specifications hence this is of grave concern because of its well established toxicological implications. Agricultural runoffs, domestic effluent discharge, refuse dump runoff and runoff from car wash and garage were the main anthropogenic sources of pollution in the river. Consequently, based on the overall results of this study, Nairobi River is polluted and its water is not safe for domestic purposes including drinking notwithstanding the fact that the concentration of COD and TSS were generally high and above the NEMA limits for natural water.. Stricter environmental regulation for water quality is needed so as to support the protection and management strategies of Nairobi River and prevent further deterioration of the river.

ACKNOWLEDGEMENTS:

The authors acknowledge valuable contributions of the National Council of Science, Technology and Innovation (NACOSTI) and the Nairobi City Water and Sewerage Company.

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Received on 25.09.2016 Modified on 10.10.2016

Asian J. Research Chem. 2016; 9(11): 579-586.

DOI: 10.5958/0974-4150.2016.00078.X

parameter Sampling Point	Mn	Fe	Pb	Zn	Cu	Cr
CKK	0.5061	0.4121	0.009	0.0195	0.1099	0.0423
KKK	0.3944	0.3300	0.2522	0.0090	0.0755	nd
RR	2.471	0.7806	nd	nd	0.0704	nd
LS	1.2579	2.0913	0.1814	0.0021	0.0781	0.0165
LD	1.3522	1.9168	0.0237	0.0032	0.0675	0.0376
KN	0.0991	0.2348	nd	nd	0.0813	nd